1. Field of the Invention
The present application relates to a lens system for a collimated optical beam to enhance the optical coupling efficiency to an optical waveguide.
2. Background Arts
A technique to form a planar optical circuit (PLC) by semiconductor materials has been popular in the field. A conventional PLS is typically made of dielectric material such as lithium niobate (LiNbO3). Because the refractive index of most semiconductor materials is greater than that of LiNbO3, the PLC of semiconductor materials reduces not only a plane size thereof but a cross section thereof. The optical waveguide with a mesa structure has effective dimensions determined by a width of the mesa and a thickness of a waveguide layer. The former dimension extends to several micron-meters (μm) but the latter is forced to be limited to sub-micron meters.
Conventionally known technique to enhance the optical coupling efficiency to the waveguide is to provide a spot-size converter (SSC) in a vicinity of an end surface of a mother material on which the optical waveguide exposes. The SSC may equivalently expand the cross section of the waveguide. However, an SSC inherently has complicated structures to form, which raises a production cost of the optical waveguide.